1. Technical Field
The present invention relates to a shift control apparatus for a continuously variable transmission installed in a vehicle. Particularly, the present invention relates to improvement in shift control during sudden braking of the vehicle.
2. Related Art
In a known belt CVT (continuously variable transmission) as disclosed in, for example, JP-A No. 2006-97757 or JP-A No. 2010-242935, a gear shift is made by increasing or reducing the individual widths of pulley grooves of a pair of pulleys around which a belt is winded.
Specifically, while the belt is clamped by two pairs of sheaves, each pair constituting one pulley, in order to apply tension to the belt, the clamped state of the belt is changed by the sheaves constituting at least one or both of the pair of pulleys in order to make a gear shift. For example, a hydraulic actuator for the secondary pulley at the output side of the CVT is supplied with a hydraulic pressure meeting a required torque, such as typified by an engine load, to ensure a necessary amount of transmission torque (control of belt clamping force). On the other hand, a hydraulic actuator for the primary pulley at the input side of the CVT is supplied with a hydraulic pressure for a gear shift to change the winding position of the belt (control of gear ratio).
During sudden braking of a vehicle equipped with a conventional belt CVT, for the purpose of ensuring the re-accelerability or startability later on, the hydraulic pressure of the hydraulic actuator for the primary pulley, regarding the control of gear ratio, is controlled to increase the gear ratio (change it toward γmax). Concurrently, as regards the control of belt clamping force, the hydraulic pressure of the hydraulic actuator for the secondary pulley is controlled to increase the belt clamping force. In this case, the hydraulic pressure to be supplied to the hydraulic actuator for the secondary pulley is fixedly set to increase the shift speed of a gear shift toward γmax. In other words, in order to early achieve a gear ratio reflecting a change in vehicle speed due to sudden braking (increase the belt returnability), the hydraulic pressure to be supplied to the hydraulic actuator for the secondary pulley is set a predetermined amount higher than before.
However, if during sudden braking the hydraulic pressure to be supplied to the hydraulic actuator for the secondary pulley is simply set a fixed amount higher than before, this simple measure has no way to optimize the hydraulic pressure. The reason for this is that the optimum value of the belt clamping force varies depending upon the condition of a road surface on which the vehicle is traveling. Therefore, the conventional control of belt clamping force does not have the capability to optimize the belt clamping force according to the road surface condition.
Specifically, if the aforementioned belt clamping force (set a fixed amount higher than before) is short of the optimum value, sufficient belt returnability may not be able to be obtained. On the other hand, if the hydraulic pressure is higher than the optimum value, the power of an oil pump for providing the hydraulic pressure is increased more than necessary, which will reduce the energy efficiency. More specifically, for example, in a CVT in which the hydraulic pressure discharged from an oil pump operable by the power of an engine (internal combustion engine) is used, the fuel economy will be reduced.
JP-A No. 2006-97757 discloses a technique in which the shift speed is changed depending upon the gear ratio or the deceleration of the vehicle. However, this technique cannot provide an optimum shift speed appropriate to the road surface condition.